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selecting the difference between frames

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Mak Tzoepselecting the difference between frames
by on Feb 4, 2010 at 5:25:26 pm

hello hippies
am I dreaming or have I actually seen/read about a video program (plug?) that is able to select the difference between frames and turn it into a mask?
Im not an AFX avid user (no pun), just use the program for editing and simple effects. But I could use this now: say I film a street with static objects. Then a person passes through the frame. I tell the prog/plug to select the difference, thus being able to turn the result into a mask.
Any ideas?
thanks!


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Todd KoprivaRe: selecting the difference between frames
by on Feb 4, 2010 at 5:54:53 pm

You're describing the Difference Matte effect.

"About keying: color keys, luminance keys, and difference keys":

"Difference keying works differently from color keying. Difference keying defines transparency with respect to a particular baseline background image. Instead of keying out a single-color screen, you can key out an arbitrary background. To use difference keying, you must have at least one frame that contains only the background; other frames are compared to this frame, and the background pixels are made transparent, leaving the foreground objects. Noise, grain, and other subtle variations can make difference keying very difficult to use in practice."

Note the warning in that last sentence about how difficult his can be to pull off in practice.

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Todd Kopriva, Adobe Systems Incorporated
putting the 'T' back in 'RTFM' : After Effects Help on the Web
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Dave LaRondeRe: selecting the difference between frames
by on Feb 4, 2010 at 6:11:56 pm

I've tried using the Difference Key myself, and had to give it up as a bad job: the quality of my footage just wasn't good enough. When Todd says to take the warning seriously, he isn't kidding.

I probably works well on something like 2K RED footage shot expressly for the footage, but on your normal HDV or DV, it's a train wreck. p2 footage? Iffy, depending on the camera: an HVX 200 is pretty noisy. I don't know if even Digibeta would work well.

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA


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Todd KoprivaRe: selecting the difference between frames
by on Feb 4, 2010 at 6:31:58 pm

The Difference Matte can get you started. But, for example, if you have noise or grain, then you'll have artifacts in your matte that you'll need to clean up with any of several techniques. I once got a decent matte using the Difference Matte effect after I applied the Median effect to the matte---removing speckles from the matte.

So, don't give up without trying---but also know that you will likely have some more work to do. And you can't even hope to get a good result without starting with a locked-down shot and good, clean footage with minimal (preferably zero) compression artifacts.

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Todd Kopriva, Adobe Systems Incorporated
putting the 'T' back in 'RTFM' : After Effects Help on the Web
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If a page of After Effects Help answers your question, please consider rating it. If you have a tip, technique, or link to share---or if there is something that you'd like to see added or improved---please leave a comment.


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Michael SzalapskiRe: selecting the difference between frames
by on Feb 4, 2010 at 6:56:18 pm

You know what works even if your footage is noisy? Rotoscoping!
Yay!

- The Great Szalam
(The 'Great' stands for 'Not So Great, in fact, Extremely Humble')

No trees were harmed in the creation of this message, but several thousand electrons were mildly inconvenienced.


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Walter SoykaRe: selecting the difference between frames
by on Feb 4, 2010 at 6:40:33 pm

[Mak Tzoep] "say I film a street with static objects. Then a person passes through the frame. I tell the prog/plug to select the difference, thus being able to turn the result into a mask"

If your goal is to remove the person from the shot, you might look into Boris FX Motion Key.

Walter Soyka
Principal & Designer at Keen Live
Motion Graphics, Widescreen Events, Presentation Design, and Consulting
RenderBreak Blog - What I'm thinking when my workstation's thinking
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